Logarithmic amplifier using common cathode positive compensating feedback



May 14, 1963 J. A. HAMMER LOGARITHMIC AMPLIFIER USING COMMON CATHODEPOSITIVE COMPENSATING FEEDBACK Filed June 23, 1959 PRIOR ART 3 V UOUTPUT I m? Ru U0 INPUT U gRx I b UK u I OUTPUT Ub Uo I My Thisinvention relates to an amplifier, the output voltage of which islogarithmically related to the input voltage.

Such amplifiers are for instance useful in computers for transposing alinear voltage analogue of a computer quantity into a logarithmicanalogue, whereby a conversion into decibels can be elfected.

It is known that with suitable electron tubes, operated at appropriateconditions the grid current depends logarithmically from thegrid-cathode and anode-cathode voltages.

For this purpose the tube can be used in a circuit according to FIG. 1of the drawings, in which the anode of a tube V is connected to thepositive terminal of a supply source b with a voltage U through an anoderesistor R,,, the negative terminal of said source being connected tothe cathode of the valve V through a cathode resistor R The output U isderived from the terminals 3 and 4. The grid of the tube V is connectedto the input terminal -1 via a grid series resistor R and the inputterminal 2 is directly connected to the output terminal 4. The potentialbetween the input terminals 1 and 2 is indicated by U In this circuitthe following relations apply:

From this it appears that log (U, U,, U )=log R,

s, 1., +(a U,-R,-t,-Rko,+i,)

in which U is the plate to cathode voltage of the tube V, U is the gridto cathode voltage of the tube, L; is the grid current, I, is the anodecurrent, a is a constant generally equal to the reciprocal of theamplification factor of the tube, S is a factor generally known as themutual conductance of the tube, and S is a factor generally known as theplate conductance of the tube. The term log signifies the logarithm ofthe following quantity.

It the circuit is arranged so that a=constant 1 S =constant (2) S=constant (3 e e U U U1 the equation will obtain the following form:

log U =C +C .i,,

which is a condition of the desired type.

It is possible to satisfy the conditions (1-4) by correctly adjusting asuitable tube-type.

In case U, can vary over a large range, e.g. 60 db which often occurs inpractice, the condition (5 cannot be satisfied. Since the sum of U -l- Ufor the desired logarithmic operation is in the order of 1 volt, theminimum value Patented May 14, 1963 of U should be 10 volts forobtaining an accuracy of only 10%, in which case the maximum value ofsaid potential would be 10,000 volts, which is impracticable.

By means of the circuit as described it will therefore not be possibleto obtain a reasonable accuracy over a large range.

An object of the invention is to provide a circuit in which over a largerange (60 db) a good accuracy 4 db) can be obtained.

It is an object of the invention to provide a logarithmic amplifiercircuit, the output voltage of which is an accurately logarithmicfunction of the input voltage throughout a large range of inputvoltages.

A feature of the invention is the provision of means, adding to theinput voltage such a voltage, derived from the amplifier, that thevoltage losses across all components of the input circuit with theexception of the series resistor are compensated.

A further feature is that said means consist of a second tube, thecathode of which is directly connected to the cathode of the first tube,the anode being directly connected to the supply source and the controlgrid of which being connected to the anode of the first tube through apotentiometer.

According to the invention the error in a thermionic grid controlledlogarithmic amplifier resulting from the change in voltage of the gridof the tube with respect to ground with change in input current iscorrected by applying a voltage to the cathode of the tube that issubstantially equal and opposite to said grid voltage change. Thecompensating voltage is preferably obtained from an amplifier responsiveto the anode voltage of the logarithmic amplifier.

The invention now will be described with reference to the drawings inwhich- FIG. 1 is a circuit of a known logarithmic amplifier and FIG. 2is an amplifier circuit according to the invention.

In FIG. 2 a second tube V is cathode-coupled to the first tube V. Thequantities referring to the valve V are indicated with the samereference symbols as the corresponding ones of the valve V, but providedwith a prime.

The anode of V is directly connected to the supply source b and thecontrol grid is connected with the sliding contact of a potentiometer P,connected between the anode of the valve V and the terminal 4.

As for the valve V: S U,, +S U =i,, the voltage generated across thecommon cathode resistor by the anode current is proportional to the gridvoltage U and therefore also proportional to the voltage at the slidingcontact of the potentiometer P with respect to the terminal 4. Thevoltage at said sliding contact is proportional to the voltage betweenthe anode of V and the terminal 4, the relative magnitude beingadjustable by means of the potentiometer P. The voltage U beingproportional to and opposite (in sense) to the grid voltage U,; of thetube V it follows that the voltage, generated by the tube V across thecathode resistance will be proportional to and opposite (in sense) tothe grid voltage U,, of the valve V. By means of a suitable adjustmentof the sliding contact of the potentiometer P it can be obtained thatthe voltage generated by the tube V across R will be equal but oppositein sense to the voltage, generated by the valve V across R minus thegrid voltage U of the valve V.

In this case U -l-U =0, so that the condition (5) is satisfied,regardless of the magnitude of the input voltage.

By means of the circuit it has been possible to satisfy the desiredlogarithmic relation within narrow boundaries (1% db) over a largevoltage range (60 db).

I claim:

1. A logarithmic amplifier comprising an amplifier tube having agrid-cathode circuit and an anode-cathode circuit, said grid-cathodecircuit including a series resistor connected to a source of positivesignal voltage such that grid current generally proportional to thesignal voltage flows in the grid-cathode circuit, a cathode resistorcommon to said circuits, and means responsive to the current flow in theanode-cathode circuit arranged to maintain the potential of the gridsubstantially constant as the grid current varies with the signalvoltage, whereby said grid current is proportional to said signalvoltage.

2. In a logarithmic amplifier comprising an amplifier tube having agrid-cathode circuit and an anode-cathode circuit, a cathode resistorcommon to said circuits, a resistor connecting the tubegrid to apositive signal voltage source such that the grid current is generallyproportional to the signal voltage applied to the circuit, and anamplifier that is connected to the anode-cathode circuit to beresponsive to current flow in said circuit and that is connected to saidcathode resistor to apply a voltage to the cathode of the amplifier tubetending to maintain the grid of such tube at a constant potential as thegrid current varies with the voltage applied to the grid circuit,whereby the grid current is proportional to the signal voltage.

3. A logarithmic amplifier comprising an amplifier tube having an inputcircuit that includes a resistor connected between a signal source and agrid of the tube, an output circuit connected to an anode of the tube,and a cathode resistor, said tube having an anode current that issubstantially a logarithmic function of the grid current, and meansresponsive to said output circuit adapted to apply 4 a voltage to thecathode of the tube in an amount to cancel changes in potential at thegrid of the tube resulting from changes in grid current, whereby thegrid current is determined solely by said series resistor and thevoltage from said signal source.

4. A logarithmic amplifier comprising an amplifier tube having a grid,an anode, and a cathode, a resistor connecting the grid to a source ofsignal voltages that is polarized to cause grid current flow, an outputcircuit, a cathode resistor connecting the cathode to return sides ofthe signal source and the output circuit, said tube having an anodecurrent that is a logarithmic function of the grid current, and a secondamplifier tube that is responsive to said output circuit and thatapplies a signal to the first amplifier to cancel any variations in thesum of the gridcathode and cathode resistor voltages resulting fromsignal voltages in the grid circuit.

References Cited in the file of this patent UNITED STATES PATENTS2,162,878 Brailsford June 20, 1939 2,600,423 Nolle June 17, 19522,662,213 Vanderlyn Dec. 8, 1953 2,685,000 Vance July 27, 1954 2,877,348Wade Mar. 10, 1959 2,903,524 Howell Sept. 8, 1959 OTHER REFERENCES Vol.21, MIT Radiation Laboratory Series (Electronic Instruments),McGraW-Hill, 1948, pages and 126.

Linear-to-Logarithrnic Voltage Converter," Electronics, July 1953, pages156 and 157.

I awe-U

1. A LOGARITHMIC AMPLIFIER COMPRISING AN AMPLIFIER TUBE HAVING AGRID-CATHODE CIRCUIT AND AN ANODE-CATHODE CIRCUIT, SAID GRID-CATHODECIRCUIT INCLUDING A SERIES RESISTOR CONNECTED TO A SOURCE OF POSITIVESIGNAL VOLTAGE SUCH THAT GRID CURRENT GENERALLY PROPORTIONAL TO THESIGNAL VOLTAGE FLOWS IN THE GRID-CATHODE CIRCUIT, A CATHODE RESISTORCOMMON TO SAID CIRCUITS, AND MEANS RESPONSIVE TO THE CURRENT FLOW IN THEANODE-CATHODE CIRCUIT ARRANGED TO MAINTAIN THE POTENTIAL OF THE GRIDSUBSTANTIALLY CONSTANT AS THE GRID CURRENT VARIES WITH THE SIGNALVOLTAGE, WHEREBY SAID GRID CURRENT IS PORPORTIONAL TO SAID SIGNALVOLTAGE.